1. Field of the Invention
The present invention relates to a key operated rotatable plug and cylinder lock of the type that utilizes three sets of tumbler pins arranged in three rows extending in three planes that radiate from a common axis that extends centrally through a plug-defined keyway for engaging one row of recesses formed in a narrow edge surface, land two rows of recesses formed in opposed flat side surfaces of a substantially flat key. More particularly, the present invention relates to a rotatable plug and cylinder lock of the type described wherein one of the three planes extends from the center axis in a direction that parallels the opposed, relatively flat side surfaces of the key when the key is inserted into the keyway, while the two other planes extend from the center axis in non-aligned directions and at angles of inclination relative to the first plane that are unequal, with their angles of inclination being selected from a set of angles that reside within a range of about seventy five to about one hundred five degrees, with the angles that comprise each such set differing from each other by at least about five degrees, and with the inclination angles of ninety and one hundred five degrees being avoided in selecting at least one, preferably both, of these angles.
2. Prior Art
Rotatable plug and cylinder locks are known that employ three rows of tumbler pins arrayed about a central axis of a plug-defined keyway for engaging recesses that are formed in opposite side and edge surfaces of a flat key that can be reversibly inserted into the keyway. One of the rows of tumbler pins typically engages recesses formed in an edge surface of an inserted key, while each of the other rows engages recesses formed in a separate one of the opposed flat sides of the inserted key.
A traditional approach that has been taken in arranging three rows of tumbler pins to engage recesses formed in a substantially flat key has called for a first row of tumbler pins to extend in a first plane that is disposed between and parallels the opposed flat sides of an inserted key (so that the first set of tumbler pins engages recesses formed in a narrow edge surface of the inserted key) with second and third sets of tumbler pins extending in second and third planes, respectively, that extend from the central axis on opposite sides of the first plane, at equal angles of inclination relative to the first plane (whereby a "symmetrical" arrangement of tumbler pins is defined in that the second and third sets of tumbler pins extend at equal angles relative to the first plane and therefore can be said to be arranged symmetrically about the first plane).
Two "symmetrical" arrangements of three rows of tumbler pins have gained wide acceptance for use with flat keys. One widely accepted arrangement calls for second and third sets of tumbler pins to be inclined at right angles relative to a first edge-engaging set of tumbler pins, with the first, second and third sets extending in first, second and third planes, respectively, that intersect at right angles along a center axis that extends centrally through a plug-defined keyway. With this arrangement, the second and third sets of tumbler pins are "aligned" in the sense that they extend in opposite directions within a common plane. Recesses that are formed in opposite sides of a key to receive inner ends of the tumblers are not "oblong" (as is the case when the recesses are drilled at angles of inclination other than ninety degrees), but rather are distinctively circular--which enables one who is skilled in the art to ascertain the angular relationship of tumblers in a lock by gaining only a quick glance at a key that operates the lock. This "right angle" arrangement of second and third sets of tumbler pins relative to a first set has come to be referred to as a "90/90" arrangement, with each of the numerals "90" referring to angles of inclination of second and third rows of tumbler pins relative to a first row of tumbler pins that engages a flat edge surface of an inserted key.
A more secure, somewhat more difficult to defeat arrangement of three sets of tumbler pins that has come to be widely used with a flat key calls for second and third rows of tumbler pins to be equally inclined at angles of one hundred five degrees relative to a first row of edge-engaging tumbler pins. With this arrangement, recesses that are formed in opposite flat sides of a key for receiving inner ends of the second and third rows of tumblers are of "oblong" shape inasmuch as they are drilled at one hundred five degree angles relative to the flat side surfaces of the key. A problem with this symmetrical "105/105" arrangement of tumblers is that it has come to be sufficiently well known that it ordinarily is "assumed to exist" by those who are skilled in the art when they see the characteristically "oblong" shaped recesses that are formed in opposite flat sides of a key that is used to operate a lock of this type. As those who are skilled in the art of picking locks have come to be increasingly familiar with techniques that can be used to defeat locks that employ the relatively standard 105/105 symmetrical arrangement of tumbler pins, locks that embody this tumbler pin arrangement are progressively coming to be viewed as providing less that a desired degree of security.
The 90/90 and 105/105 symmetrical arrangements of tumbler pins have come to be so widely accepted for use with flat keys that key cutting machines now are available that are designed to form recesses in opposite sides of keys that either are inclined at ninety degrees or at one hundred five degrees. Thus, when it comes to producing keys that have recesses that will accommodate pins that extend toward flat side surfaces of an inserted key at angles of either ninety or one hundred five degrees, relatively little difficulty is encountered--a factor that also tends to diminish the degree of "security" that is perceived to be associated with locks that employ 90/90 and 105/105 symmetrical tumbler pin arrangements.